An example of a typical variable compression-ratio (expansion-ratio) engine, is disclosed in Japanese Unexamined Patent Application Publication No. 2000-073804, the contents of which are incorporated herein in their entirety.
In the variable compression-ratio engine discussed in Japanese Unexamined Patent Application Publication No. 2000-073804, each rocking lever that supports a corresponding piston is linked to a crankshaft by means of a first connecting rod, such that the rocking lever revolves around a crankpin of the crankshaft. The piston reciprocates back and forth within a corresponding cylinder, and the reciprocation of the piston is converted to a rotary motion of the crankshaft via the rocking lever and the first connecting rod. The crankshaft and associated members collectively represent a revolving mechanism.
A motor separate from the engine selectively rotates an eccentric shaft, and due to the eccentricity of the eccentric shaft, the rocking lever connected to the eccentric shaft by means of a second connecting rod revolves around the crankpin of the crankshaft. The revolution of the rocking lever changes the relative position between the top dead center of the piston and the cylinder, thereby changing the compression ratio of the internal combustion engine.
The engine is equipped with a common sensor for the cylinders or a plurality of sensors provided individually for each cylinder. The common sensor or each of the sensors is configured to detect knocking or predictive knocking. Furthermore, the engine is also equipped with a controlling mechanism configured to drive the revolving mechanism while simultaneously changing the compression ratio of the cylinders. If the sensor detects knocking or predictive knocking of any one of the cylinders, the controlling mechanism simultaneously lowers the compression ratio of the cylinders.
For an example of an engine that controls opening and closing timings of intake and exhaust valves, the contents of Japanese Unexamined Patent Application Publication No. 8-177429 is incorporated herein in its entirety.
The engine of this example has a communication hole in a tubular sleeve, and a working-fluid supply/exhaust passageway that communicates with the communication hole. Moreover, the engine also has a duct arrangement, which is provided in a tappet and communicates with the communication hole. The tappet has a hydraulic chamber therein and extends slidably through the sleeve.
When the sleeve and the tappet are relatively displaced from each other, the duct arrangement and the communication hole are blocked off from each other. Then, a piston extending slidably through the hydraulic chamber is driven together with the tappet, thereby opening or closing the valves. In other words, until the communication between the duct arrangement and the communication hole is blocked off, the piston is not driven since the oil in the hydraulic chamber can enter or leave the chamber. Consequently, the engine is equipped with a driving mechanism for driving the sleeve and a controlling mechanism for controlling the driving mechanism. The controlling mechanism can be the same as or different from the controlling mechanism discussed above. Based on information received from an operating-condition detecting mechanism, the controlling mechanism drives the sleeve by a predetermined amount through the driving mechanism so as to control the distance (i.e., time) required to block off the communication between the duct arrangement and the communication hole. In this manner, the valve timing changes.
When it is determined that the variable compression-ratio engine is in operation within a low revolution range, or in a startup period, or in a cold startup period, the valve timing of the intake valve or the exhaust valve is adjusted to a retarded opening timing and advanced closing timing (i.e., a smaller operating angle). On the other hand, if it is determined that the variable compression-ratio engine is in operation within a high revolution range, the valve timing of the intake valve or the exhaust valve is adjusted to an advanced opening timing and retarded closing timing (i.e., a larger operating angle).
In an engine in which the compression ratio (expansion ratio) is adjustable, the compression ratio of the engine is generally set at maximum for a low-load operating range to reduce fuel consumption. However, the present inventors have found that if the expansion ratio is too high when the load is equal to or below a predetermined low load value, the pressure in the cylinder becomes lower than the atmospheric pressure just before the exhaust valve opens in the latter half of an expansion process. In other words, the engine does negative work, leading to high fuel consumption. This is especially prominent in a case where the compression ratio becomes substantially lower than the expansion ratio, which is caused when the valve timing of the intake valve is set to a retarded opening timing and advanced closing timing such that the closing timing of the intake valve is significantly advanced from the bottom dead center point.
A variable compression-ratio engine such as that disclosed in Japanese Unexamined Patent Application Publication No. 2000-073804, and incorporated herein in its entirety is intended to avoid the occurrence of knocking by reducing the compression ratio when knocking occurs in a high-load operating range. On the other hand, the engine applies a high compression ratio (expansion ratio) for a low-load operating range in order to improve the fuel consumption.
On the other hand, in an engine such as that disclosed in Japanese Unexamined Patent Application Publication No. 8-177429, and incorporated herein in its entirety, which controls the opening and closing timings of the intake and exhaust valves, the control of the opening timing of the exhaust valve is intended for increasing the gas exchange efficiency for the high revolution range (i.e., absorb a large amount of new gas by reducing as much residual gas as possible). However, Japanese Unexamined Patent Application Publication No. 8-177429 has no description with regard to a control operation for improving the fuel consumption under low load.